Methods for the transmission of data between a plurality of resource-constrained devices and a non-geostationary satellite and associated system

ABSTRACT

A method for the transmission to a non-geostationary satellite of data, each device storing data, carried out by each device, includes performing successive Doppler shift estimations including receiving, by the device, a signal from the satellite comprising at least one elevation emission criterion, performing a Doppler shift estimation based on the frequency of the received signal; estimating a Doppler rate of frequency change to obtain a position of the device relative to the position of the satellite, defining a transmission window during which the position of the device relative to position of the satellite verifies the elevation emission criterion, defining a transmission time included in the transmission window and defined randomly, emitting, at the transmission time, a signal including data stored by the device and the position of the device relative to the position of the satellite.

TECHNICAL FIELD

The technical field of the invention is the field of communications insystems comprising non-geostationary satellites and ground-baseddevices.

The present document concerns methods for the transmission of databetween a resource-constrained device and a non-geostationary satelliteand associated system, and particularly wherein the non-geostationarysatellite is a low earth orbit (“LEO”) satellite and whereincommunications are enhanced by leveraging the use of a beacon signal ofthe LEO satellite.

STATE OF THE ART

With the development of Internet of Things (“IoT”) networks andLow-Power Wide-Area Networks (“LPWAN”), Low Earth Orbit satellitesconstellations have been found to be of interest as they particularlywell integrate with such systems. Indeed, LEO satellites allow a globalcoverage starting from just one satellite. LEO satellites constellationsare for example used for data collection, to retrieve data from parts ofLPWAN or IoT networks and to deliver the data to other parts of saidnetworks.

Most of the time, these constellations are used to retrieve data fromground-based objects unable to communicate with the rest of the networkdue to their location, and to deliver said data to ground-based stationsof the network to make the data accessible by the rest of the network.The objects emit messages to the LEO satellites when in radio visibilityof a satellite and the LEO satellites emit the messages to theground-based stations when in radio visibility of a ground-basedstation. The LEO satellites constellations are therefore used as relaysin the systems comprising the constellation and a LPWAN or IoT network.

In the majority of these systems, such as Argos®, Orbcomm® etc., thecoverage offered by the LEO constellation is not constant, meaning thatthe object does not always have a LEO satellite in radio visibility inorder to transmit its messages.

In order to be able to transmit, the object needs to predict the futurepassage of a satellite in the constellation and often also needs to knowits relative position within the satellite visibility to transmit withinthe radio coverage of the system and to optimize the radio transmissionas the satellite visibility is not equal to its radio coverage.

Existing systems rely on constellation ephemerides to predict thepassage of the LEO satellites and time windows for transmission. The useof ephemerides has two main disadvantages.

First, to be able to predict the next passage of the LEO satellite, itis necessary that the transmitting object also knows its own position,which implies:

-   -   In the case of a fixed transmitter object, to indicate to the        transmitter object its position when it is put into service,    -   In the case of a mobile transmitter object, to add a radio        positioning system (e.g. of the geo-positioning system “GPS”        type). The addition of a GPS has a significant impact on the        cost of the radio module of the object and on battery        consumption of the object, which is a critical characteristic of        LPWA transmissions by nature for which transmitters must be able        to transmit messages for several years with very limited power        source.

A second disadvantage is that it is necessary to transmit ephemeris tothe object on a regular basis (specifically for satellites orbiting at500 to 600 km altitudes, due to the imprecision on the modelling of thedrag perturbation). The transmitter needs a refresh of the ephemeris:

-   -   with a frequency of the order of magnitude of few weeks to        ensure that the error on the passage estimate is sufficiently        reliable, and/or    -   with a frequency of the order of magnitude of the day if it is        necessary to estimate a short window such as the satellite        visibility time for the object.

To transmit an updated ephemeris, it is necessary:

-   -   either that the transmitter downloads this information from a        source external to the satellite (e.g. via a terrestrial        network) and thus for the mobile object to be in visibility of        the terrestrial network sufficiently regularly. In many        applications this is not the case. For example, in the case of        maritime transport, containers can spend several weeks/months at        sea.    -   either that the LEO satellite broadcasts these ephemerides via a        communication channel to the objects. The problem with this        solution is that it has a significant impact on the LEO        satellite, requiring the implementation of a transmission        channel of several hundred bits per second. Moreover, the        reception of these ephemerides by the object may consume a        significant amount of power and therefore impacts the autonomy.

Another problem of mobile ground-based objects is linked to thereception frequency of the LEO satellite. The frequencies used varygeographically from country to country. For example, the ISM bands inthe United States (902-921 MHz) are different from those used in Europe(868-870 MHz), so hybrid objects communicating via satellite must beprovided with central frequency and bandwidth information. For mobileobjects this requires to know its position (via a positioning modulesuch as GPS) and to have a table referencing the regulatory frequencies.In the same way as for ephemerides, it is currently necessary for theobject to be covered by or connected to a terrestrial network in orderto download or update this information.

A major problem is that current systems require ground-based objects tobe equipped with a GPS for collecting and having information on theposition of an object and to transfer this information. This requirementhas an impact on the cost and energy consumption of the ground-basedobject. This position data can further be intercepted by an entity thatis not the intended recipient of the information.

Another problem is a collision problem, linked with the distribution ofthe ground-based objects in the radio coverage of the satellite: whenseveral of the objects want to emit at the same time, there is a risk ofcollision of the signals, and therefore a problem may arise where theLEO satellite does not receive all of the data from all of the objects.There is no solution in the state of the art that permits to reduce thecollisions while requiring low energy consumption from the objects.

Another problem is that, in the ISM (industrial, scientific and medical)bands, the level of interference will vary with time. The noise floorwill raise in dense radio areas (e.g. when the satellite passes over bigcities). This could result in a reduction of the area over which thelink budget closes. In such cases, the objects within the radioreception coverage but located at the edge of the coverage will try totransmit their frame but the frames will not be decoded by the satelliteas the link budget does not close (i.e. does not result in appositivelink margin). This results in a degraded Quality of Service (QoS).

There is therefore a need for a solution to enable ground-based devicesto send data to a satellite with reduced collisions while requiring lessenergy and having a low cost of the ground-based device.

SUMMARY OF THE INVENTION

The present document solves the above-mentioned problems by providingsolutions for a plurality of low-cost ground-based devices to be able totransmit data with reduced collisions at emission to a non-geostationarysatellite while requiring low power.

According to a first aspect of the invention, this is satisfied byproviding a method for the transmission to at least onenon-geostationary satellite of a plurality of pieces of data, eachground-based device of a plurality of ground-based devices storing atleast one piece of data of the plurality of pieces of data, carried outby each ground-based device of the plurality of ground-based devicescarrying out the method, the method comprising the following steps:

-   -   Performing successive Doppler shift estimations, a Doppler shift        estimation of the Doppler shift estimations comprising:        -   Receiving, by the ground-based device, at least one signal            from the non-geostationary satellite, the signal comprising            at least one elevation emission criterion,        -   Performing a Doppler shift estimation based on the frequency            of the received signal;    -   Estimating a Doppler rate of frequency change to obtain a        position of the ground-based device relative to the position of        the non-geostationary satellite, the estimation being carried        out by using an estimation method on a plurality of computed        Doppler rates of frequency change, the computing of a Doppler        rate of frequency change comprising deriving the Doppler shift        estimations,    -   Defining a transmission window, the transmission window being a        period of time during which the position of the ground-based        device relative to position of the non-geostationary satellite        verifies the elevation emission criterion,    -   Defining a transmission time, the transmission time being        comprised in the transmission window and being defined randomly,    -   Emitting, at the transmission time, at least one signal        comprising at least the piece of data stored by the ground-based        device and the position of the ground-based device relative to        the position of the non-geostationary satellite.

Thanks to the invention, by only using a randomisation of the emissionin the transmission window, the probability of collision is greatlyreduced and independent of the distribution of the ground-based devices10 in the radio coverage.

The method according to the invention may also have one or more of thefollowing characteristics, considered individually or according to anytechnically possible combinations thereof:

-   -   the received signal from the non-geostationary satellite further        comprises at least one time reference and the transmission time        is defined by implementing a slotted ALOHA scheme using the time        reference.    -   the received signal from the non-geostationary satellite further        comprises at least one information of the current altitude of        the non-geostationary satellite at the time of emission of the        signal and the current altitude information is used in the step        of defining a transmission window to define an adjusted        transmission window.    -   the elevation emission criterion is an information of the        current beam size of the non-geostationary satellite or an        elevation emission threshold

Another aspect of the invention relates to a communication system forthe transmission to at least one non-geostationary satellite of aplurality of pieces of data, each ground-based device of a plurality ofground-based devices storing at least one piece of data of the pluralityof pieces of data, wherein each ground-based device of the plurality ofground-based devices is configured to carry out the method according tothe invention.

The communication system according to the invention may also have one ormore of the following characteristics, considered individually oraccording to any technically possible combinations thereof:

-   -   the non-geostationary satellite is a low Earth orbit satellite        or a medium Earth orbit satellite.    -   the ground-based device is a resource-constrained device.

Another aspect of the invention relates to a computer program productcomprising instructions which, when the program is executed by acomputer, causes the computer to carry out any one of the methodsaccording to the invention.

Another aspect of the invention relates to a computer-readable mediumcomprising instructions which, when executed by a computer, cause thecomputer to carry out any one of the methods according to the invention.

The invention finds a particular interest when the ground-based devicesare mobile and have no connection possible to a ground-based network.Indeed, the invention permits to resources-constrained ground-baseddevices to have enough autonomy to transmit data to non-geostationarysatellites for long periods of time without having to maintainephemerides and without relying on GPS while being able to changeposition, always having an accurate positioning and avoiding collisionsduring emission.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will become clearfrom the description that is given thereof below, by way of indicationand in no way limiting, with reference to the appended figures, amongwhich:

FIG. 1 is a schematic representation of a communication system accordingto the invention,

FIG. 2 is a schematic representation of a method according to theinvention carried out by a ground-based device,

FIG. 3 is a schematic representation of a step of the method accordingto the invention carried out by the ground-based device,

FIG. 4 is a schematic representation of a method according to theinvention carried out by a non-geostationary satellite,

FIG. 5 is a schematic representation of exchanges between a ground-baseddevice carrying out the method of FIG. 2 and a non-geostationarysatellite carrying out the method of FIG. 4 ,

FIG. 6 is a schematic representation of another method according to theinvention carried out by a ground-based device,

FIG. 7 is a schematic representation of another method according to theinvention carried out by a non-geostationary satellite,

FIG. 8 is a schematic representation of another method according to theinvention carried out by a non-geostationary satellite,

FIG. 9 is a schematic representation of another method according to theinvention carried out by a ground-based device,

FIG. 10 is a schematic representation of the broadcasting of regionalinformation by a non-geostationary satellite and the reception of theinformation by a ground-based device located in another region,

FIG. 11 is a schematic representation of the broadcasting of regionalinformation by a non-geostationary satellite and the reception of theinformation by two ground-based devices,

FIG. 12 is a schematic representation of the broadcasting of regionalinformation by a non-geostationary satellite and the reception of theinformation by two ground-based devices.

DETAILED DESCRIPTION

For greater clarity, identical or similar elements are marked byidentical reference signs in all of the figures.

FIG. 1 presents the communication system 1 implementing the methodsaccording to the invention. The communication system 1 comprises anon-geostationary satellite 20 and a ground-based device 10.

The non-geostationary satellite 20 is a satellite orbiting a celestialbody such as the Earth. When the non-geostationary satellite 20 orbitsthe Earth, it can then be a low Earth orbit satellite (with an orbitaltitude below 2000 km) or a medium Earth orbit satellite (with an orbitaltitude above 2000 km and below geosynchronous orbit). Thenon-geostationary satellite 20 comprises at least a processor and amemory, the memory storing instructions which, when executed by theprocessor, cause the processor to carry out steps of the methodsdescribed later. The non-geostationary further comprises at least onecommunication payload, the communication payload comprising at least onetransponder configured to receive and send signals towards the celestialbody. The signal can be sent in spot beams of different width, forexample or 60 degrees (satellite beam half Field of View).

The ground-based device is a device comprising at least a processor anda memory, the memory storing instructions which, when executed by theprocessor, cause the processor to carry out steps of the methodsdescribed later. Preferably, the ground-based device 10 is a mobiledevice, meaning that the device is able to be moved for example byhaving a low weight or by comprising means making it able to be moved.Preferably, the ground-based device 10 is a resource-constrained device.It is understood by a resource-constrained device a device comprising alimited processor and/or limited memory such as a low-cost processorand/or a low-cost memory or such as an Internet of Things device, ableto connect to an IoT network such as a Sigfox® network, a Lora® networkor any other network permitting to link IoT devices. Further, theground-based device comprises at least a communication module configuredto communicate, that is to receive and send signals, at least with thenon-geostationary satellite 20. Further, the communication module of theground-based device 10 can be configured to communicate through aground-based IoT network or through any other network, via wired meansor wirelessly, for example towards a ground-based station 30. Tocommunicate through a ground-based IoT network or through any othernetwork, via wired means or wirelessly, for example towards aground-based station 30, the ground-based device 10 can comprise anothercommunication module. In the example represented at FIG. 1 , theground-based device 10 is unable to reach the ground-based station 30directly or to reach a network permitting to reach the ground-basedstation 30. The ground-based device 10, preferably resource-constrained,is able to reach the non-geostationary satellite 20 using at least oneof the methods according to the invention described in the presentdocument.

A first method according to the invention is represented FIG. 2 . Themethod M1a represented FIG. 2 is carried out by the ground-based device10 and comprises six steps.

In a first step 11, the ground-based device 10 enters an awaken mode.The ground-based device 10 can for example have several functioningmodes, such as an awaken mode and an asleep mode. In an asleep mode, theground-based device 10 can be unable to receive communications and thussaves energy. The ground-based device 10 in an asleep mode can havelimited processing abilities. In an awaken mode, the ground-based device10 is in active search of a signal from the non-geostationary satellite20. The awaken mode is entered at periodic times, for example for 5seconds every minute. That way, the ground-based device 10 saves energycompared to the state of the art and is still able to communicate withthe non-geostationary satellite 20. When in awaken mode, theground-based device listens for at least one signal from thenon-geostationary satellite 20. If the ground-based device 10 does notreceive at least one signal during the time it is in awaken mode, forexample during the 5 seconds, it goes back to the asleep mode for apredetermined period of time.

In a step 12, successive Doppler shift estimations are performed. ADoppler shift estimation comprises two sub-steps 121 and 122.

First, in a sub-step 121 of reception, the ground-based device 10receives, during the time the ground-based device 10 is in awaken mode,at least one signal from the non-geostationary satellite 20. This signalwill be hereinafter referred to equally as “beacon signal” or as“signal”. The received signal comprises at least one frequency parameterof the current reception frequency used by the non-geostationarysatellite 20 and at least one elevation emission criterion.Advantageously, the invention permits to use both the fact that a signalis received to estimate a Doppler shift and the content of the receivedsignal to enhance the abilities of the ground-based device 10 whilelimiting its energy consumption.

This reception follows the emission of the signal by thenon-geostationary satellite 20 at a step 21 of the method M2arepresented at FIG. 4 . In the method M2a, the step of emitting thesignal towards the ground-based device 10 is performed periodically bythe non-geostationary satellite 20, the signal comprising the at leastone frequency parameter of the current reception frequency used by thenon-geostationary satellite 20 and the at least one elevation emissioncriterion. This step is for example repeated periodically for 1 secondevery 5 seconds to save energy on the non-geostationary satellite 20side but can also be performed continuously. When performed continuouslyon the satellite side, as opposed to for 1 second every 5 seconds, theground-based device 10 can also listen 5 times less. Thus, thisparameter must be chosen by taking into account both the power onboardthe non-geostationary satellite 20 and the autonomy of the ground-baseddevice 10.

It will be understood by “frequency parameter” any indication enablingthe ground-based device 10 to obtain the current reception frequencyused by the non-geostationary satellite 20 at the time of emission ofthe signal. The current reception frequency preferably comprises thecentral frequency and the channel width to be used. The indication ispreferably encoded in bits in the signal. The number of bits depends onthe total number of possible reception frequencies used by thenon-geostationary satellite 20. For example, for 8 different receptionfrequencies (or for 8 different radio configurations), only 3 bits canbe used, and for 16 different reception frequencies, 4 bits can be used.When receiving the indication, the ground-based device 10 checks forexample a lookup table where a number in bit format is linked to a givenfrequency that should be used and a channel width. For example, a firstconfiguration could be a European configuration, where the centralfrequency is 868.4 MHz and a channel width is 400 kHz. In anotherconfiguration, for example in Asia, the central frequency can be 902 MHzwith a channel width of 400 kHz. In another embodiment, the parametercan be directly comprised in the signal, by inserting in the signal thefrequency that should be used when emitting to the non-geostationarysatellite 20 and the channel width. This removes the need for afrequency lookup table in the ground-based device 10 but makes thesignal data larger. The invention covers any other way of formattingthis information for transmission to the ground-based device 10 in thebeacon signal or for transmission to the ground-based device 10 usingthe beacon signal.

It will be understood by “elevation emission criterion” any indicationenabling the ground-based device to obtain a criterion defining aminimum elevation above the ground-based device 10 that thenon-geostationary satellite 20 should verify for the ground-based device10 to emit to the non-geostationary satellite 20. When receiving theindication, the ground-based device comprises for example a lookup tablewhere a number in bit format is linked to an elevation threshold. Theindication is preferably encoded in bits in the signal. The number ofbits can depend on the number of possible radio configurations. Thepossible radio configurations can depend on the width of thenon-geostationary satellite 20 beam. For example, two possibleconfigurations can be “Narrow” for a narrow beam and “Wide” for a widebeam. When receiving the indication, the ground-based device 10 checksfor example a lookup table where a configuration number in bit format islinked to a given elevation threshold that should be used to emit to thenon-geostationary satellite 20. An elevation threshold can be linked toeach configuration. For example, for a wide beam configuration, anelevation threshold of degrees can be specified while for a narrow beamconfiguration an elevation threshold of 45 degrees can be specified. Inanother embodiment, the elevation criterion can be directly comprised inthe signal, by inserting in the signal the elevation threshold thatshould be used when emitting to the non-geostationary satellite 20. Thisremoves the need for a frequency lookup table in the ground-based device10 but makes the signal data larger. The invention covers any other wayof formatting this information for transmission to the ground-baseddevice 10 in the beacon signal or for transmission to the ground-baseddevice 10 using the beacon signal.

In a sub-step 122, the ground-based device 10 performs a Doppler shiftestimation based on the frequency of the received signal. Such a Dopplershift, in Hz, is retrieved by computing the difference between areceived frequency of a continuous wave emitted in the signal, and theexpected frequency at emission of the continuous wave stored by theground-based device 10.

As the ground-based device 10 is preferably a resource-constraineddevice, it performs the previous Doppler shift estimation using low-costor low-quality oscillators. As such, the Doppler shift estimationsperformed by the ground-based device 10 are heavily biased, introducingerrors that can exceed the total amplitude of the Doppler shift. Thebias is typically of around 20 ppm while the Doppler shift varies of+/−23 ppm for an altitude of 550 km. The invention permits to overcomethat problem by performing successive Doppler shift estimations in astep 12, as represented FIG. 2 and FIG. 5 , and by estimating Dopplerrates of change based on a plurality of frequency Doppler rates ofchange based on the successive Doppler shift estimations.

The successive Doppler shift estimations can be performed on a fixedperiod basis, such as for example every 5 seconds, or on an irregularperiod basis, such as for example spaced of 5 seconds, then spaced of 20seconds, then spaced of seconds, then spaced of 5 seconds, or forexample five measures spaced of 5 seconds, then to wait for 30 secondsand then again five measures of 5 seconds etc. The irregular periodbasis is more efficient than the regular period as, for a fixed numberof Doppler shift measurement, i.e. a same energy consumption, it is moreeffective to carry out for example five measures spaced of 5 seconds,then to wait for seconds and then to carry out again five measures of 5seconds, rather than carrying out ten measures spaced of 5 seconds.Indeed, in the first case, the covered satellite orbital arc is largerthan in the second case, and so the estimation precision of the geometryof the pass above the ground-based device 10 is improved, for the sameenergy consumption in the first and second case.

As represented at FIG. 5 , the non-geostationary satellite 20 emits, ina step 21, a beacon signal comprising at least a frequency parameter andan elevation criterion parameter towards the celestial body it orbits.The beacon signal is received by the ground-based device 10, whichperforms step 12 of the method M1a, i.e. it performs a Doppler shiftestimation based on the frequency of the received signal. Theground-based device 10 then stores the Doppler shift performed for lateruse in step 13 of estimation of a Doppler rate of frequency change.

In step 13, represented at FIG. 2 in the method M1a and at FIG. 5 , theground-based device performs an estimation of a Doppler rate offrequency change to obtain a position of the ground-based device 10relative to the position of the non-geostationary satellite 20, theestimation being carried out by using an estimation method on aplurality of computed Doppler rates of frequency change, the estimationof a Doppler rate of frequency change comprising deriving the Dopplershift measures. For example, for N Doppler shift estimation f1, . . .,fN performed at times t1 to tN, the ground-based device 10 computes N−1derivatives by computing:

$\frac{{df}_{k}}{dt} = \frac{f_{k + 1} - f_{k}}{t_{k + 1} - t_{k}}$

These Doppler shift derivatives (i.e. Doppler rates of frequency change)are then used as an entry in an estimation method, such as for example aleast-squares estimation method, to determine the geometry of thecurrent pass of the satellite, defined by its maximum elevation at peak,and the position of the ground-based device temporally in that currentpass. This is carried out by creating abacuses (i.e. one or severalmatrices), the abacuses giving the value of the Doppler shift as afunction of the maximum elevation at peak and of the time before peak.The ground-based device then interpolates in these abacuses using theestimation method, to obtain the values of maximum elevation at peak andtime before peak from successive values of Doppler rates. Said abacusesare obtained from the following method:

$\left( \frac{- 1}{c} \right)*\frac{R_{e}*R_{sat}*{\sin\left( {\omega*t} \right)}*{\cos\left( {{\cos^{- 1}\left( {R_{e}*\frac{\cos\left( \theta_{\max} \right)}{R_{sat}}} \right)} - \theta_{\max}} \right)}*\omega}{\sqrt{R_{e}^{2} + R_{sat}^{2} - {2*R_{e}*R_{sat}*{\cos\left( {\omega*t} \right)}*{\cos\left( {{\cos^{- 1}\left( {R_{e}*\frac{\cos\left( \theta_{\max} \right)}{R_{sat}}} \right)} - \theta_{\max}} \right)}}}}*f$

With Re the radius of the celestial body, R_sat the addition of thealtitude of the satellite and of the radius R of the celestial body, ωthe angular velocity of the satellite in a celestial body centered fixedframe, in rad/s, θ_max the pass maximum elevation in radians, t the timebefore peak, c the speed of light, and f the frequency.

Thus, a position of the ground-based device 10 relative to thenon-geostationary satellite 20 is obtained.

By applying an estimation method on the derivatives of Doppler shifts(i.e. Doppler rates of frequency change), the errors introduced by thelow-cost oscillator(s) are removed, the only value impacting the resultbeing the Doppler estimation variance.

Further, at a step 14 of the method M1a, the ground-based device 10defines a transmission window, the transmission window being a period oftime during which the position of the ground-based device 10 relative toposition of the non-geostationary satellite 20 verifies the elevationemission criterion. Using the estimated current pass of thenon-geostationary satellite 20 and the position of the ground-baseddevice 10 temporally in the current pass, a window can be defined bydeducing a start and end instants of a temporal window for which theelevation is above the elevation emission criterion, using for examplethe following equation to determine the elevation of thenon-geostationary satellite 20 as a function of time:

${\theta(t)} = {\cos^{- 1}\frac{R_{sat}*{\sin\left( {\gamma(t)} \right)}}{\sqrt{R_{sat}^{2} - {2*R_{e}*R_{sat}*{\cos\left( {\gamma(t)} \right)}} + R_{e}^{2}}}}$

Where γ(t) is the distance between the nadir of the non-geostationarysatellite 20 and the ground-based device 10 at a time t when representedin a spherical triangle as described in [“Doppler Characterization forLEO Satellites”, Irfan Ali et al, March 1998]. The spherical trianglecomprises three vertices M, N and P, where γ(t_0) is the distancebetween the vertices M (nadir of the non-geostationary satellite 20 atmaximum elevation) and P (the ground-based device 10) and γ(t) is thedistance between the vertices N (nadir of the non-geostationarysatellite 20 at time t) and P (the ground-based device 10). The sidebetween the vertices M and N is the angular distance measured on thesurface of the celestial body along the ground trace from the time t tothe time t_0 where the non-geostationary satellite 20 is at maximumelevation for the ground-based device 10.

γ(t) can be computed using the following formula:

${\gamma(t)} = {\cos^{- 1}\left( {\frac{{cos\gamma}_{0}}{\cos\left( {\omega*\left( {t_{v} - t_{0}} \right)} \right)}*{\cos\left( {\omega*\left( {t - t_{0}} \right)} \right)}} \right)}$where ${cos\gamma}_{0} = \frac{R_{e}}{R_{sat}}$

With t_0 is the instant at maximum elevation, t_v is the instant whenthe non-geostationary satellite 20 just becomes visible to theground-based device 10, and γ_0 is the distance between the nadir of thenon-geostationary satellite 20 and the ground-based device 10 at instantt_v.

Finally, R_sat can be found using the following formula:

${R\_{sat}} = \frac{\sqrt{\begin{matrix}{{\left( {R_{e}*{cos\theta}_{\max}} \right)^{2}*{\cos^{2}\left( {\omega*\left( {t_{v} - t_{0}} \right)} \right)}*\left( \frac{1}{{sin\theta}_{\max}} \right)^{2}} - {2*}} \\{\left( {R_{e}*{cos\theta}_{\max}} \right)*{\cos\left( {\omega*\left( {t_{v} - t_{0}} \right)} \right)}*R_{e}*\left( \frac{1}{{tan\theta}_{\max}} \right)*} \\{\left( \frac{1}{{sin\theta}_{\max}} \right) + {R_{e}^{2}*\left( \frac{1}{{tan\theta}_{\max}} \right)^{2}} + R_{e}^{2}}\end{matrix}}}{\cos\left( {\omega*\left( {t_{v} - t_{0}} \right)} \right)}$

At a step 15, a transmission time in the transmission window is defined.The transmission time can be defined randomly, as will be describedlater in another embodiment, or as the first time at which the positionof the ground-based device 10 verifies the elevation criterion, or asthe time at a predetermined fixed delay after the beginning of thetransmission window, or any other definition.

In a step 16, the ground-based device 10 verifies that, at thetransmission time comprised in the transmission window, the position ofthe ground-based device relative to the position of thenon-geostationary satellite 20 verifies the elevation emissioncriterion. This checking can be performed using the previously computedrelative position of the ground-based device 10 to the non-geostationarysatellite 20, using the previously Doppler shift measurements, oranother estimation can be performed using a lower number of new Dopplershift derivatives, and Doppler rate computation. For example, this canbe done using 3 received signals.

In a step 17 of the method M1a, the ground-based device 10 emits, at thetransmission time and at the current reception frequency received in thebeacon signal, at least one signal comprising at least the piece of datastored by the ground-based device 10. The signal emitted by theground-based device 10 at step 17 can further comprise the position ofthe ground-based device 10 relative to the position of thenon-geostationary satellite 20 as estimated at steps 13 or 16.

In another embodiment, the ground-based device 10 advantageously takesinto account the fact that it is in an occulted environment or notbefore emitting at step 17. To do so, the ground-based device 10 carriesout the step 17 of emission only if the ground-based device 10 receivesanother signal from the non-geostationary satellite 20 during a timeperiod before the transmission time. This time period can be for exampleof 10 seconds. This permits to enhance the Quality of Service (QoS) byonly emitting when possible and thus reducing the emissions resulting inno reception or a degraded reception by the non-geostationary satellite20.

In a step 22 of the method M2a, and as represented at FIGS. 4 and 5 ,the non-geostationary satellite 20 receives the signal sent by theground-based device 10 at the current reception frequency sent in thebeacon signal, the signal comprising the at least one piece of datastored by at least one ground-based device 10. The received signal canalso comprise the position of the ground-based device 10 relative to theposition of the non-geostationary satellite 20 and a timestamp relatedto the time at which the position of the ground-based device 10 relativeto the position of the non-geostationary satellite 20 has beenestimated. This permits to locate the ground-based device 10 and/or toassociate a location to the received piece(s) of data at the time ofestimation of its position by the ground-based device 10. As thenon-geostationary satellite 20 knows its absolute position, or as aground-based satellite station 40 does so, the non-geostationarysatellite and/or the ground-based satellite station 40 is able tocompute the absolute position of the ground-based device 10 based on theposition of the ground-based device 10 relative to the position of thenon-geostationary satellite and on the absolute position of thenon-geostationary satellite 20.

Then, when the non-geostationary satellite 20 passes over a ground-basedsatellite station 40, the non-geostationary satellite 20 transmits theat least one piece of data to the ground-based station 40 as representedFIG. 5 .

In another embodiment of the method M1a, the beacon signal sent by thenon-geostationary satellite 20 and received by the ground-basedsatellite 20 further comprises at least one information of the currentaltitude of the non-geostationary satellite 20 at the time of emissionof the signal. In this embodiment, the current altitude information canbe used in the step 14 of defining a transmission window to define anadjusted transmission window. As shown before, the altitude intervenesin the computing of the abacuses. Using the altitude information, theground-based device can select the best fitted abacus, if several abacihave been implemented in the ground-based device 10. This choice is atrade-off between complexity and performance. The altitude alsointervenes in the computation of visibilities durations and elevations.Taking into account the altitude in both cases permits to lower theestimation errors, and therefore to take less margin in estimating thetransmission window. An adjusted transmission window is a transmissionwindow better fitted for transmission, that is for example a largertransmission window to avoid signal collisions between the signals ofdifferent ground-based devices 10 emitting at the same time or duringoverlapping periods.

In another embodiment of the method M1 a compatible with the embodimentwhere the beacon signal comprises an altitude information, theground-based device uses an enhanced wake-up method and the step 11 ofentering an awaken mode comprises two sub-steps 111 and 112 asrepresented at FIG. 3 .

In a first sub-step 111, a reduced wake-up time window is computed bythe ground-based device 10. The computing of a reduced wake-up timewindow comprises computing orbital period of the non-geostationarysatellite 20 using previous received signals from the non-geostationarysatellite 20 at sub-step 121 during other passes of thenon-geostationary satellite 20.

The reduced wake-up time window computing comprises three sub-steps:

-   -   First, a computing or retrieving of the orbital period. This        sub-step can be carried out using the altitude comprised in the        beacon signal in the previous embodiment and/or using a        peak-to-peak algorithm described later. When using the altitude,        the orbital period can be found using the following formula:

$T = {2*\pi*\frac{\sqrt{\left( R_{sat} \right)^{3}}}{\mu}}$

with T the non-geostationary satellite 20 orbital period, p thecelestial body standard gravitational parameter (the product of thegravitational constant of the celestial body and its mass), R_sat theradius of the non-geostationary satellite 20, computed by adding thealtitude of the non-geostationary satellite 20 with the radius R_e ofthe celestial body the non-geostationary satellite 20 is orbiting.

-   -   Secondly, a determination of the peak of the current pass. This        can be done using the defining of a transmission window at step        14, or by observing the change of sign of the Doppler shift        (when the Doppler shift is null, it means that the        non-geostationary satellite 20 is at the peak of its current        pass).    -   Thirdly, an application of the orbital period to estimate the        next pass at the determined peak (with an error margin). When        applying the orbital period to determine the time of the next        pass, the following points must be taken into account:        -   If the non-geostationary satellite 20 passes from an            ascending (from South to North) (respectively descending,            from North to South) to an ascending (respectively            descending) pass, the difference between two peaks is equal            to one orbital period.        -   If the non-geostationary satellite 20 passes from an            ascending (respectively descending) to a descending            (respectively ascending) pass, the difference between two            peaks is no longer equal to one orbital period but to            (1+f1)*orbital period (respectively (1+f2)*orbital period).            These fractions f1 and f2 are computed using the            peak-to-peak algorithm described later.

The peak-to-peak algorithm will now be described.

At each pass of the non-geostationary satellite 20 at the peak, theground-based device 10 stores the time and associates it to thenon-geostationary satellite 20. Then, from all the stored times, theground-based device 10 determines the orbital period. It also determinesthe f1 and f2 fractions previously mentioned:

-   -   To obtain the orbital period, the ground-based device 10 first        divides a delta time between peaks by a hardcoded orbital period        estimate value. The hardcoded value is an orbital period known        of a satellite for example at 550 km altitude for the first        computation. For example, at initialization, the ground-based        device 10 uses an orbital period of 95.6 minutes for a satellite        at an altitude of 550 km.        -   If the computation is carried out between two passes from an            ascending pass to an ascending pass or from a descending            pass to a descending pass, this division gives substantially            an integer. By dividing the delta time between peaks by this            obtained integer a plurality of times for different delta            times between peaks and by computing the mean value of the            result, a precise value for the orbital period is found.        -   If the computation is carried out between two passes from an            ascending pass to descending pass or from a descending pass            to an ascending pass, the division performed several times            and the computing of the mean values gives the fractions f1            (from an ascending pass to descending pass) and f2 (from a            descending pass to an ascending pass).

In order to obtain the orbital period, only the passes from an ascendingpass to an ascending pass or from a descending pass to a descending passcan be used if enough passes are available to perform the computation.If there are not enough passes, all the passes can be used to obtain theorbital period which then takes into account the passes from anascending pass to descending pass or from a descending pass to anascending pass.

Once the next pass or passes is/are known, the ground-based device 10defines reduced time windows during which to wake-up, drasticallyreducing the power consumption. From an initial time TO at which thenon-geostationary satellite 20 is above the ground-based device 10, theground-based device 10 must then wake-up:

-   -   from an ascending pass to an ascending pass or from a descending        pass to a descending pass: at a time T0+T, T being the orbital        period    -   from an ascending pass to a descending pass: at a time T0+T*f1    -   from a descending pass to an ascending pass: at a time T0+T*f2

To even further reduce the number of time windows during which towake-up, the beacon signal emitted by the non-geostationary satellite 20can further comprise an information related to the type of pass(ascending or descending pass).

In a sub-step 112, the ground-based device 10 only enters the awakenmode during the defined reduced time window. This advantageously allowsto reduce the energy consumption of the ground-based device 10 by onlysearching for a signal from the non-geostationary satellite 20 inreduced time windows during which the non-geostationary satellite 20 ispassing close to the ground-based device 10.

A second method according to the invention is represented FIG. 6 . Themethod M1b represented FIG. 6 is carried out by a plurality ofground-based devices and comprises five steps. Each ground-based device10 carries out the method on its own, the method M1b enabling to reducecollisions between signals when at least two ground-based devices 10want to emit to the non-geostationary satellite 20 while being both inthe radio coverage of the non-geostationary satellite 20 at the sametime or during overlapping periods.

Steps 12, 13, 14 and 17 of the method M1b are the same as steps 12, 13,14 and 17 of method M1a. The method M1 b does not comprise the step 16of checking if, at a transmission time comprised in the transmissionwindow, the position of the ground-based device 10 relative to theposition of the non-geostationary satellite 20 verifies the elevationemission criterion.

Indeed, in the method M1b, the transmission time is randomly defined.The invention of method M1b advantageously relies on the fact that byusing a randomisation of the emission in the transmission window, theprobability of collision is greatly reduced and becomes independent ofthe distribution of the ground-based devices 10 in the radio coverage ofthe non-geostationary satellite 20. This is particularly advantageouswhen a significant number of ground-based devices 10 need to emit in thesame limited area.

Therefore, the step 15 of method M1b is the step 15 of the method M1a ofdefining a transmission time, the transmission time being comprised inthe transmission window, but the step 15 of method M1b is limited to thetransmission time being defined randomly compared to the step 15 of themethod M1a. This permits to remove the need for synchronisation betweenground-based devices 10 when they both want to emit in the radiocoverage of the non-geostationary satellite 20.

Preferably, a slotted ALOHA scheme is implemented. The slotted ALOHAscheme needs a time reference to be implemented, to define slots of timeat the start of which the ground-based devices 10 should try to emit.This time reference is therefore comprised in the signal sent by thenon-geostationary satellite 20. For example, a time reference can be thereception time by the ground-based device 10 of the first bit of the“data” part of the signal sent by the non-geostationary satellite 20.The slotted ALOHA scheme defines slots of a predetermined duration, andonly allows ground-based devices to emit at the start of each slot. Thisallows to further reduce the risks of collisions when two or moreground-based devices 10 want to emit in the radio coverage of thenon-geostationary satellite 20. The Slotted ALOHA scheme can beimplemented together with the randomization of the transmission time,the time slot of emission in the slotted ALOHA scheme being definedrandomly.

The method M1b can further comprise the step 11 of the method M1a ofentering an awaken mode, and the enhanced awakening mode, therebyreducing the energy consumption for each ground-based device 10.

As in the method M1a, the step 14 of defining a transmission window canfurther use an altitude information to define an adjusted transmissionwindow.

A third method according to the invention is represented at FIG. 7 . Themethod M2b represented at FIG. 7 is carried out by a non-geostationarysatellite 20 and comprises four steps. The method M2b comprises thesteps 23 of performing periodically at least one radio coveragemeasurement and 24 of computing a dynamic elevation emission criterionbased on the last radio coverage measurement performed, and the steps 21and 22 of the method M1b of respectively emitting a signal and receivinga signal.

In a first step 23, the non-geostationary satellite 20 performsperiodically at least one radio coverage measurement. The radio coveragemeasurement can be performed non-stop. It can also be performed for aperiod long enough to determine that the state of the observed system issufficiently stationary to last in the future, for example dozens ofseconds. The radio coverage measurement can for example comprise aspectrum analysis, a noise floor measurement and an interferencesmeasurement, for example using an SDR payload (for “Software DefinedRadio” payload), for example at 14 dBm of 100 Hz (100 bits per second)in the ISMs bands close to 868 MHz.

When the radio coverage measurement of step 23 is performed, thenon-geostationary 20 defines an elevation criterion or updates anexisting elevation criterion in a step 24. This definition at step 24can comprise for example performing a link budget computation.

A link budget computation comprises estimating the link margin indecibels for different elevations between the ground-based device 10 andthe non-geostationary satellite 20. For example, for a minimal elevationof 20 degrees and no interfering noise, at 14 dBm of 100 Hz (100 bitsper second) in the ISM band close to 868 MHz, the link margin can be of2.5 dB, meaning that it is possible for the non-geostationary satellite20 to receive a signal emitted by the ground-based device 10 with suchan elevation. Therefore, the non-geostationary satellite 20 can set thedynamic elevation criterion to a value of 20 degrees. But, a minimalelevation of 20 degrees and interfering noise, at 14 dBm of 100 Hz (100bits per second) in the ISM band close to 868 MHz, the link margin canbe of −5.2 dB, meaning that the non-geostationary satellite 20 would notreceive a signal emitted by the ground-based device 10 with such anelevation. The next minimum elevation with a positive link margin is anelevation of 40 degrees with a link margin of 0.8 dB. Thenon-geostationary satellite 20 can then set the dynamic elevationcriterion to a value of 40 degrees, resulting in a useful coveragereduced by 1150 km, i.e. nearly 75% compared to the case withoutinterfering noise, but ensuring that no frames are lost and thereforethat the coverage and Quality of Service match as closely as possiblethe current environment.

The description made here considers a homogeneous noise level over theentire 200 kHz channel. One could very well understand that theinvention further covers when the beacon transmits different elevationcriteria per band slice (e.g. 50 kHz or 10 kHz) depending on the noiselevel measured in each of the slices.

When a dynamic elevation emission criterion has been set, thenon-geostationary satellite 20 performs the steps 21 and 22 of themethod M2a, that is it emits the signal comprising the elevationemission criterion periodically and, if the position of thenon-geostationary satellite 20 relative to the position of theground-based device 10 verifies the dynamic elevation emissioncriterion, it receives at least one signal sent by the ground-baseddevice 10, the signal comprising at least one piece of data stored by atleast one ground-based device 10. The emission of the beacon signal bythe non-geostationary satellite 20 can be performed periodically whilethe elevation emission criterion does not change, and when it has beenchanged, continue to emit periodically the new elevation emissioncriterion.

Thanks to the method M2b, the elevation emission criterion is dynamic,that is the elevation emission criterion comprised in the signal sentperiodically towards the Earth changes as a function of the radiocoverage measurement carried out by the non-geostationary satellite 20.This allows to have a better Quality of Service as the ground-baseddevice 10 uses an elevation criterion that is coherent with the realcurrent radio coverage of the non-geostationary satellite 20.

A fourth method according to the invention is represented FIG. 8 . Themethod M2c represented FIG. 8 is carried out by a non-geostationarysatellite 20 and comprises two steps.

The method M2c is a method for the transmission of a region-specificpiece of information from at least one non-geostationary satellite 20 toat least one ground-based device 10, the method M2c being carried out bythe non-geostationary satellite and comprising a least the step 21 ofemitting at least one signal periodically towards the Earth, the signalcomprising the region-specific piece of information and an elevationcriterion, the region-specific piece of information being specific to aregion around the nadir of the non-geostationary satellite, theelevation criterion being relative to the region around the nadir of thenon-geostationary satellite.

In an embodiment, the region-specific piece of information in the signalemitted at step 21 of the method M2c is a regional reception frequencyof the satellite 20, to be used by the ground-based device 10 whenemitting to the non-geostationary satellite 20 when the ground-baseddevice 10 is in the region around the nadir of the non-geostationarysatellite 20. When the region-specific piece of information is aregional reception frequency of the satellite 20, the method M2c cancomprise another step 22 of, if the position of the non-geostationarysatellite 20 relative to the position of the ground-based device 10verifies the elevation emission criterion, receiving at least one signalsent by the ground-based device 10 at the regional reception frequencyof the non-geostationary satellite 20.

In an embodiment, when the nadir of the non-geostationary satellite 20changes region, the region-specific piece of information comprised inthe signal changes. This applies to a regional reception frequency.

The region around the nadir is defined by performing a trade-off betweenan information refresh rate of the region-specific piece of informationand a size of incertitude of the region-specific piece of information.It is understood by “information refresh rate” a rate at which theregion-specific piece of information will be considered valid by theground-based device 10. For example, the transmission rate can be of twohours, meaning that the ground-based device 10 is considered to havereceived a valid piece of information every two hours. When performingthe trade-off, the “information refresh rates” are evaluated fordifferent elevations, for example 70, 75, 80 and 85 degreesrespectively, corresponding to an incertitude (or region) centeredaround the Nadir point with a diameter of 351 km, 259 km, 171 km and 85km (corresponding diameters for a non-geostationary satellite 20orbiting at an altitude of 525 km).

The results show that the smaller the accuracy sought in the size of theregion around the nadir, the higher the transmission rate.

The signal emitted by the non-geostationary satellite 20 can furthercomprise the current reception frequency of the non-geostationarysatellite 20. This is represented at FIGS. 10 to 12 .

The example represented at FIG. 10 comprises a non-geostationarysatellite 20 with a reception frequency set as “frequency 1”. Thenon-geostationary satellite 20 passes over region A for which theregulatory frequency is frequency 1. The ground-based device 10 is inradio coverage of the non-geostationary satellite 20 and, as shown inthe method M1c represented at FIG. 9 , in a step 18, the ground-baseddevice 10 receives the beacon signal comprising two pieces ofinformation related to frequencies: the reception frequency Frequency 1of the non-geostationary satellite 20, and the regulatory frequency in aregion around the nadir of the position of the non-geostationarysatellite 20, which is also Frequency 1 at FIG. 10 . The beacon signalcan further comprise, with the regulatory frequency, a regionalelevation criterion, i.e. a criterion defining the size of the regionaround the Nadir. The elevation criterion can alternatively bedetermined or evaluated by the ground-based device 10. In a step 19, ifthe position of the non-geostationary satellite 20 relative to theposition of the ground-based device 10 verifies the regional elevationcriterion, that means that the ground-based device 10 is inside thedefined region around the Nadir (the hatched area of FIG. 10 ), so theground-based device 10 defines its emission frequency as being theregional frequency, i.e. frequency 1 in FIG. 10 . As shown in FIG. 10with the arrow, this is particularly advantageous if the ground-baseddevice 10 moves from the region B to the region A, and if theground-based device 10 is configured to emit at frequency 2 of region B,when it should use the frequency 1, as it is the regulatory frequency ofregion A. As the ground-based device 10 is then in the region around thenadir of the non-geostationary satellite 20, it can set the reglementaryfrequency to frequency 1 as it verifies the regional elevationcriterion. The invention permits to maintain a correct regionalinformation in the ground-based device 10 by only defining the regionalinformation as “correct” in the ground-based device 10 when the positionof the ground-based device 10 relative to the non-geostationarysatellite 20 verifies the regional elevation criterion. Indeed, theregional elevation criterion defines the hatched region around the nadirof the non-geostationary satellite 20. In this embodiment, it isimportant to note that the regional elevation criterion and theelevation emission criterion are two different elevation criteria: theregional elevation criterion defines the region around the Nadir of thenon-geostationary satellite 20 and is used by the ground-based device 10to store the region-specific piece of information if the ground-baseddevice 10's position verifies the regional elevation criterion, whilethe elevation emission criterion is used for the ground-based device 10to know if it is condition (i.e. well positioned) to emit to thenon-geostationary satellite 20.

Preferably, and as represented at FIG. 9 , the position will be obtainedas in the method M1a, thus leveraging the use of the beacon signal, andlimiting the energy consumption of the ground-based device 10 to obtaina position. It will be understood that the invention described in steps18 and 19 is not limited to the acquisition of the position using step12 to 16, but to any way of acquiring the position of the ground-baseddevice 10.

As represented at FIG. 9 , the method M1c comprises the step 12 ofestimating successive Doppler shifts, and the step 13 of estimating aDoppler rate of frequency change to obtain a position of theground-based device 10 relative to the position of the non-geostationarysatellite 20. When a position has been obtained, the step 19 can beperformed. Further, optionally, the steps 14 to 16 can be performed forthe ground-based device 10 to emit using the internally set regulatoryfrequency.

As represented at FIG. 11 , the ground-based device 10 a is in theregion around the nadir of the non-geostationary satellite 20. Thereception frequency of the satellite is set to Frequency 2 and theregulatory frequency, i.e. the region-specific piece of information, isFrequency 1. In this embodiment, the non-geostationary satellite 20advertises the regional reception frequency as well as its own receptionfrequency. When the ground-based device 10 a receives the beacon signalfrom the non-geostationary satellite 20, it checks if its positionverifies the regional elevation criterion, which is the case, as it ispositioned in the cone in the hatched area of FIG. 10 . Therefore, instep 19, the ground-based device 10 a defines its frequency as theregional frequency “frequency 1”. After, it checks if the currentreception frequency received in the beacon is equal to the currentreception frequency of the non-geostationary satellite 20, which is thennot the case. Therefore, the ground-based device 10 a does not emit itsmessage. The position of the ground-based device 10 b does not verifythe regional elevation emission criterion (i.e. it is not in the hatchedarea), so it does not set its frequency as the regional frequency.However, the ground-based device 10 b has its emission frequency set atFrequency 2 and therefore can emit to the non-geostationary satellite 20as the non-geostationary satellite 20 has a reception frequency also setat Frequency 2. Such cases where the current reception frequency set inthe non-geostationary satellite 20 and the regional frequency aredifferent can happen for example when the non-geostationary satellite 20is going to cross a border between a first region it is in and a secondregion it will be in and the distribution of ground-based devices 10 ismore important in the second region.

As represented at FIG. 12 , the ground-based device 10 a is not locatedin the region around the nadir (i.e. in the hatched area) of thenon-geostationary satellite and has its emission frequency set to theregional frequency of region A: frequency 1. As the non-geostationarysatellite has a reception frequency set to Frequency 2, the ground-baseddevice 10 a cannot emit to the non-geostationary satellite 20, eventhough the position of the ground-based device 10 a verifies theelevation emission criterion. The ground-based device 10 b has itsemission frequency set to Frequency 2. It can emit to thenon-geostationary satellite 20 as its emission frequency is the same asthe reception frequency of the non-geostationary satellite 20 and theground-based device 10 b also verifies the elevation emission criterion.If the ground-based device 10 b moves from region B, where the regionalfrequency is frequency 2, to the region A, where the regional frequencyis frequency 1, the ground-based device 10 b still has its emissionfrequency set to Frequency 2. It can thus still emit to thenon-geostationary satellite 20, as their frequencies are the same. But,the ground-based device 10 b is forbidden to use the frequency 2 as itsemission frequency should now use the frequency 1 as it is in the regionA. The invention permits to solve this problem. To do so, as theground-based device 10 b enters the hatched area, i.e. the region aroundthe Nadir of the non-geostationary satellite 20, the position of theground-based device 10 b now verifies the regional elevation criterionand now sets its emission frequency as the regional frequencybroadcasted in the beacon signal by the non-geostationary satellite 20.Its newly set emission frequency is now the regional frequencyFrequency 1. The problem is solved, as the ground-based device 10 bhaving changed regions now has the correct regional frequency set as itsemission frequency, i.e. Frequency 1. Having solved this problem, theground-based device 10 b cannot emit to the non-geostationary satellite20 as their frequencies differ, which is the expected behaviour.

1. A method for transmission to at least one non-geostationary satelliteof a plurality of pieces of data, each ground-based device of aplurality of ground-based devices storing at least one piece of data ofthe plurality of pieces of data, carried out by each ground-based deviceof the plurality of ground-based devices carrying out the method, themethod comprising: performing successive Doppler shift estimations, aDoppler shift estimation of the Doppler shift estimations comprising:receiving, by the ground-based device; at least one signal from thenon-geostationary satellite, the signal comprising at least oneelevation emission criterion, performing a Doppler shift estimationbased on the frequency of the received signal; estimating a Doppler rateof frequency change to obtain a position of the ground-based devicerelative to the position of the non-geostationary satellite, theestimation being carried out by using an estimation method on aplurality of computed Doppler rates of frequency change, the computingof a Doppler rate of frequency change comprising deriving the Dopplershift estimations, defining a transmission window, the transmissionwindow being a period of time during which the position of theground-based device relative to position of the non-geostationarysatellite verifies the elevation emission criterion, defining atransmission time, the transmission time being comprised in thetransmission window and being defined randomly, and emitting, at thetransmission time, at least one signal comprising at least the piece ofdata stored by the ground-based device and the position of theground-based device relative to the position of the non-geostationarysatellite.
 2. The method according to claim 1, wherein the receivedsignal from the non-geostationary satellite further comprises at leastone time reference and the transmission time is defined by implementinga slotted ALOHA scheme using the time reference.
 3. The method accordingto claim 1 wherein the received signal from the non-geostationarysatellite further comprises at least one information of the currentaltitude of the non-geostationary satellite at the time of emission ofthe signal and the current altitude information is used in the definingof a transmission window to define an adjusted transmission window. 4.The method according to claim 1, wherein the elevation emissioncriterion is an information of the current beam size of thenon-geostationary satellite or an elevation emission threshold.
 5. Acommunication system for the transmission to at least onenon-geostationary satellite of a plurality of pieces of data, eachground-based device of a plurality of ground-based devices storing atleast one piece of data of the plurality of pieces of data, wherein eachground-based device of the plurality of ground-based devices isconfigured to carry out the method according to claim
 1. 6. Thecommunication system according to claim 1, wherein the non-geostationarysatellite is a low Earth orbit satellite.
 7. The communication systemaccording to claim 1, wherein the ground-based device is aresource-constrained device.
 8. A non-transitory computer programproduct comprising instructions which, when the instructions areexecuted by a computer, causes the computer to carry out the methodaccording to claim
 1. 9. A non-transitory computer readable mediumcomprising instructions which, when executed by a computer, cause thecomputer to carry out the method according to claim 1.